Use of hollow spherical shells as fuel containers in the inertial confinement fusion (ICF) field has demonstrated a number of advantages well recognized in the art. In the case of glass shells in particular, these advantages include transparency, high strength, high permeability to hydrogen-isotope fuel at moderate temperatures and low permeability at room temperature. These and other advantages are also demonstrated to a greater or lesser extent by shells of polymeric or ceramic composition. One essential requirement for any fuel-containing shell in ICF applications is wall uniformity--i.e., uniform spherical geometry, wall thickness and composition. These requirements have been met in manufacture of microspheres (diameter less than about 1 mm), particularly in the case of glass microspheres. See U.S. Pat. Nos. 4,017,290 and 4,021,253. However, the requirement of wall uniformity is not well met for larger fuel containers, such as glass macrospheres of diameter on the order of about 1 mm or larger blown by hand or machine.
It has heretofore been proposed to provide macro-sized shells of the described character in a multistep operation in which hemispherical cavities are machined into two glass disks, and the disks are then assembled into a cylindrical crucible. The crucible containing the glass is rotated and heated in a horizontal tube furnace to fuse the disks to each other, and to allow surface tension to spheridize the void formed by the two hemispherical cavities. Rotating the tube at proper angular velocity time-averages the gravity vector to zero, thus simulating a zero gravity condition. The affects of gravity must be canceled to eliminate buoyancy that would effectively deform the void. After cooling, excess glass is machined away. See 1988 KMSF Annual Technical Report, pages 40-43.
Although the process so described has been successfully employed in manufacture of millimeter-size shells, improvements remain desirable. In particular, the described process, involving multiple forming and machining operations, is expensive to implement, and requires attention of highly skilled technicians to form shells successfully of desired size and uniformity. The described process is not readily amenable to economical mass production of shells. Furthermore, evacuating the shells and/or filling the shells with desired fuel material involves additional steps beyond those employed for shell manufacture.
It is a general object of the present invention to provide a method of making hollow spherical shells of uniform geometry and composition that is particularly well suited for manufacture of millimeter-size shells and larger, that is less expensive to implement than processes heretofore employed for manufacture of millimeter-size shells, that may be employed for manufacture of shells of glass, ceramic, metal or plastic composition, that yields shells of desired size and uniformity with little or no post-forming machining operations, and in which the atmosphere of the shell interior may be selectively controlled during the manufacturing process and without requiring additional process steps to evacuate the shell interior and/or fill the shell interior with selected gases.